Some communications devices derive power remotely (e.g., from relatively distant power sources through network cables) while other communications devices derive power locally (e.g., through local power supplies). For example, some voice-over-IP (VoIP) systems include a combination of remotely powered devices (e.g., IP phones, etc.) and locally powered devices (e.g., routers, switches, hubs, IP phones, etc.).
There are different approaches to remotely powering VoIP devices. In one approach, “mid-span” power injector devices are positioned on the lines (or cables) leading to the remotely powered devices, but not are positioned on the lines leading to the locally powered devices. The power injector devices allow communications signals (e.g., Ethernet signals) to pass therethrough, and simultaneously provide power on the lines without disturbing the communications signals (e.g., by using previously unused sets of twisted pair wires, by using used sets of wires, etc.).
In another approach, a particular data communications device (e.g., an Ethernet switch) is configured to (i) provide a power signal on the lines leading to the remotely powered devices and (ii) not provide the power signal on the lines leading to the locally powered devices. Accordingly, the remotely powered devices receive the power signal from the data communications device, and the locally powered devices do not receive any power signal from the data communications device.
One conventional Ethernet switch includes a set of supervisory circuit boards and multiple line cards. The set of supervisory circuit boards run software in order to discover whether devices on the other ends of the lines connected to the line cards are remotely powered or locally powered. The set of supervisory circuit boards directs the line cards to provide a power signal to the remotely powered devices, but not to provide the power signal to the locally powered devices. If someone unplugs a remotely powered device from a particular line, the set of supervisory circuit boards can discover the removal of the remotely powered device (e.g., by sensing the particular line) and subsequently direct the line card for that line to no longer provide the power signal to that line. Accordingly, if a locally powered device is later plugged into the same line, there is no power signal on that line that could otherwise damage a locally powered device.